Conventional orthogonal frequency division multiplexing (OFDM) networks and orthogonal frequency division multiple access (OFDMA) network are able to improve the reliability of the channel by spreading and/or coding data traffic and control signals over multiple orthogonal subcarriers (i.e., tones). Different user devices (i.e., subscriber stations, mobile stations, etc.) are allocated different sets of subcarriers (or tones) for transmitting and receiving data and control signals. The subcarrier frequencies are orthogonal to each other, thereby minimizing interference between user devices.
OFDM/OFDMA techniques are particularly advantageous in multiple-input, multiple output (MIMO) wireless networks that employ multiple antennas (i.e., Smart antennas) to transmit OFDM signals to the user devices. MIMO transmission schemes use multiple transmit antennas and multiple receive antennas to improve the capacity and reliability of a wireless communication channel. A MIMO system provides a linear increase in capacity with the factor K, where K is the minimum of number of transmit (M) antennas and receive (N) antennas (i.e., K=min(M,N)).
For example, four different data streams may be transmitted separately from the four transmit antennas. The transmitted signals are received at the four receive antennas. Some form of spatial signal processing is performed on the received signals to recover the four transmitted data streams. An example of spatial signal processing is V-BLAST, which uses successive interference cancellation to recover the transmitted data streams. Other variants of MIMO systems include schemes that perform some kind of space-time coding across the transmit antennas (e.g., D-BLAST) and also beam-forming techniques, such as spatial division multiple access (SDMA).
Good channel estimation is important to the performance of a MIMO transmission scheme. MIMO channel estimation consists of estimating the channel gain and phase information for links from each of the transmit antennas to each of the receive antennas. The channel for an M×N MIMO system consists of an M×N matrix:
                    H        =                  [                                                                      a                  11                                                                              a                  12                                                            …                                                              a                                      1                    ⁢                    N                                                                                                                        a                  21                                                                              a                  22                                                            …                                                              a                                      2                    ⁢                    N                                                                                                      ⋮                                            ⋮                                            …                                            ⋮                                                                                      a                                      M                    ⁢                                                                                  ⁢                    1                                                                                                a                                      M                    ⁢                                                                                  ⁢                    2                                                                              …                                                              a                  MN                                                              ]                                    [                  Eqn          .                                          ⁢          1                ]            where aij represents the channel gain from transmit antenna i to receive antenna j. In order to enable the estimations of the elements of the MIMO channel matrix, separate pilots may be transmitted from each of the transmit antennas.
Spatial multiplexing MIMO schemes can generally be divided into two broad categories: i) single stream or single codeword (SCW) schemes, and ii) multiple streams or multiple codeword (MCW) schemes. In the case of a single stream (or single-code word) MIMO transmission, a cyclic redundancy check (CRC) value is added to a single information block and then coding and modulation is performed on the block. The coded and modulated symbols are then demultiplexed for transmission over multiple antennas or beams to a single receiving device (e.g., subscriber stations, mobile station).
In the case of a multiple stream (or multiple code word) MIMO transmission, the information block is demultiplexed into smaller information blocks. Individual CRC values are attached to each of these smaller information blocks and then separate coding and modulation is performed on each of the smaller information blocks. These smaller information blocks are then transmitted from separate MIMO antennas or beams to multiple receiving devices.
It should be noted that in multiple code word MIMO transmissions, different modulation techniques (e.g., BPSK, QPSK, 16-QAM) and coding techniques (e.g., turbo coding, convolutional coding) may be used on each of the individual streams, resulting in a so-called PARC (per antenna rate control) scheme. Also, multiple code word transmission allows for more efficient post-decoding interference cancellation, because a CRC check may be performed on each of the individual streams (or code words) before each stream (or code word) is cancelled from the overall signal. In this way, only correctly received streams or code words are cancelled, thereby avoiding interference propagation in the cancellation process.
In conventional OFDMA (or OFDM) wireless networks that use MIMO transmission techniques, a data stream or codeword is transmitted from the same antenna, beam or virtual antenna over all the subbands or subcarriers allocated to the receiving device (e.g., subscriber station). However, in a multipath frequency selective channel, different subbands fade differently, thereby resulting in signal variations within codeword symbols. This degrades the system performance and capacity.
Therefore, there is a need for improved OFDMA (or OFDM) transmission systems that are capable of transmitting a stream (or codeword) to a receiving device without suffering performance degradation related to different amounts of multipath frequency selective fading in different subbands.